The present invention generally relates to distortion eliminating circuits for eliminating distortion from signals and particularly to a circuit for eliminating a harmonic distortion from a high frequency information signal reproduced from an information recording disk.
The assignee of the present invention proposed previously in the U.S. Pat. application No. 873,407, U.S. Pat. No. 4,803,677, an information recording disk on which an information signal such as a video signal is recorded by an optical beam in a form of intermittent row of pits along a recording track and a recording and reproducing apparatus for recording and reproducing the information signal on and from such a disk by means of the optical beam. When recording a video signal on the disk, a high frequency video signal comprising a frequency converted carrier chrominance signal and a frequency modulated luminance signal is produced in the recording and reproducing apparatus, and this high frequency video signal is used to drive a laser diode. The laser diode produces a high power optical beam for recording responsive to the driving by the high frequency video signal and the recording of the high frequency video signal is made on the surface of the optical disk as a row of pits by evaporating a portion of the optical disk by the optical beam.
When to reproduce the video signal from the disk, a low power optical beam or optical probe is produced by the laser diode and the optical probe scans the surface of the optical disk. On the basis of the reflection of the optical probe from the optical disk, the high frequency video signal is reproduced, and this high frequency video signal is separated into the frequency converted carrier chrominance signal and the frequency modulated luminance signal by filtering. The frequency converted carrier chrominance signal thus reproduced is processed in a color singal reproducing circuit while the frequency modulated luminance signal is first converted into a series of rectangular pulse signals having a pulse width corresponding to the frequency of the frequency modulated luminance signal and the pulse signals thus produced are then processed in a luminance signal demodulating circuit for recovering the original luminance signal.
As described previously, the information is recorded on the surface of the optical disk by irradiating the high power optical beam so as to evaporate a portion of the optical disk. Thus, the row of pits carrying the information is formed as a result of evaporation. The row of pits defines the track on the disk and the pit has a length in the direction of the track which changes responsive to the waveform of the high frequency video signal to be recorded such that the length of the pit is long when a relatively low frequency video signal is recorded and that the length of the pit is short when a relatively high frequency video signal is recorded. Further, the row of pits is formed with such a format that a length of one pit and a separation between that pit and a neighboring pit are identical. In order to achieve such a pattern of the pits, the high frequency signal which drives the laser diode comprises a series of recording pulse signals each of which consisting of a positive pulse portion and a negative pulse portion wherein the duration of the positive pulse portion and the duration of the negative pulse portion are made identical. In the description hereinafter, a signal having such a waveform will be referred to as a signal having a 50% duty cycle. It should be noted that the duration of the each pulse signal constituting the high frequency video signal is changed but the ratio in the duration of the positive pulse portion and the negative pulse portion in each pulse signal is not changed. In other words, the duty cycle of the high frequency signal which drives the laser diode is maintained at 50%. When the high frequency video signal is reproduced on the basis of the pattern of such pits formed properly on the surface of the disk, the reproduced high frequency signal reproduced by scanning the pits by the optical beam has a sinusoidal wave form. The reproduced high frequency signal having the sinusoidal wave form is then supplied to a wave processing circuit comprising a comparator and the original high frequency video signal comprising the rectangular pulse signals is recovered.
It should be noted, on the other hand, that the length of the pit on the disk is changed responsive to the power of the optical beam. Thus, the length of the pit becomes smaller than the separation between the pits when the recording is made with an optical beam having an insufficient power and the length of the pit becomes larger than the separation between the pits when the recording is made with an optical beam having an excessive power. It should be noted that when the length of the pit and the separation between an adjacent pair of pits including the aforesaid pit in the pair is not identical, the waveform of the reproduced high frequency signal is not sinusoidal but contains a harmonic distortion. Thus, when the frequency modulated luminance signal is separated from the reproduced high frequency video singal containing the harmonic distortion, the harmonic distortion is transferred to the frequency modulated luminance signal in a form of a distortion component and such a distortion component cannot be removed even if the frequency modulated luminance signal is converted to a row of rectangular pulse signals by passing through the wave processing circuit comprising the comparator. In other words, the duty cycle of the frequency modulated luminance signal thus processed is different from 50%. Thus, when the luminance signal is recovered from the row of rectangular signals representing the frequency modulated luminance signal, the luminance signal thus obtained is not identical to the original luminance signal and there occurs a carrier leak in which a high frequency carrier produced as a result of distortion is superposed on the reproduced picture. As a result, fine stripes appear in the reproduced picture and the quality of the reproduced picture is deteriorated. Further, such a reproduced luminance signal reproduced from the frequency modulated luminance signal containing the harmonic distortion affects the operation of a non-linear emphasis circuit used in the recording and reproducing circuit for removal of the noise components and the quality of the reproduced picture is further deteriorated.